This pattern of incidents over the past 10 days highlights a new wave of crackdowns that Chinese officials have launched to forestall any hint of unrest, as they brace for a string of politically sensitive anniversaries this year.

Chinese political reformers, who had hoped for a more relaxed atmosphere after the 2008 Olympic Games went well, have been disappointed.

"Things are going backwards," complains Hu Xingdou, the economics professor whose website was closed last month because local Internet censors in the city of Suzhou, near Shanghai, suddenly objected to articles that had been up on the site for more than a year without causing a stir.

"I hope that what's been happening is related to 2009, and that we won't continue to slide backwards in the future," he adds.

Tiananmen's 20th anniversary looms

Chinese leaders appear nervous about two key anniversaries this year. On June 4, 1989, Chinese troops fired on protesting students in Tiananmen Square, killing hundreds and crushing prospects for serious political reform in China. On Oct. 1, the government will celebrate the 60th anniversary of the People's Republic of China.

"The government is especially sensitive and alert," says Sun Wenguang, the retired professor who had three ribs broken by thugs on Saturday.

Professor Sun, a courageous and forthright democracy activist, was attacked in Jinan, the capital of Shandong Province, as he went to mark Chinese Tomb Sweeping day at the grave of Zhao Ziyang, the reformist prime minister who was purged from the ruling Communist Party for opposing the Tiananmen crackdown.

Although five police cars had followed him to the cemetery, Sun said from his hospital bed, and were parked 50 yards from the spot where he was attacked, "the police hid somewhere and did not stop the violence."

Sun said he had paid his respects at Mr. Zhao's tomb every year since 2005 to commemorate a man who has been erased from official Chinese history. He was physically prevented from doing so on Saturday, he speculated, because this year the authorities "are afraid there will be a big democratic movement, so they have stepped up the repression."

Routine criticism now censored

Meanwhile the Beijing News, one of the capital's most widely read tabloids, has discovered that even a cautiously worded editorial about the need for citizens to be able to evaluate their government is excessively strong meat in the current atmosphere.

The editorial, published on Monday, voiced no opinions that are not routinely debated in intellectual circles here. But by Tuesday morning it had disappeared from the newspaper's website, apparently ordered down by government censors.

Though local government officials, not Beijing, appear directly responsible for the beating of Sun, the censorship of the Beijing News, and the closure of his website, says Hu, "central government is insisting that local governments maintain social stability and prevent anything that might upset it."